Three years ago, I was working on my libretto for The Lion's Face, an opera about dementia. Feeling a little downcast by the material one day, I resolved that my next libretto would be light, sweet and comic. Then John Fulljames of the Opera Group wondered if I might take a look at Milton's Paradise Lost. I asked if a light, sweet, comic approach to the fall of man might work, at which point he wondered if I could also have a think about climate change, depletion of the planet's resources and the end of the world.

Two things, however, made this an offer I couldn't refuse. First, the involvement of the composer Luke Bedford; and second, a week rereading Milton. I say "rereading", even though my first read was probably an idle teenager's desperate flickthrough an hour before having to discuss it with an English professor. "What was your response to the poem, Mr Maxwell?" "Well, I mean just – wow!" "Go on."

To be fair, my response to it 30 years on had not especially evolved. I mean just – wow! Wow in that the lines are like nothing else in English: higher, grander, stranger. Wow in its force, authority and humanity. And wow as in: "This is impossible to do as an opera." Sung lines cannot carry the freight and complexity of verse like Milton's. It has its own intricate, deep-echoing music, like English played on some magnificent organ; it strikes those chords over silence. Sung lines have to be lighter, plainer; the vowels take on more work.

John said let's take our minds off all this and have a little chat about the end of the world. So he, Luke and I took a trip to Wakehurst Place, the offshoot of Kew Gardens near East Grinstead, where the Millennium Seed Bank, a deep and soundless Noah's Ark, endeavours to safeguard the Earth's rare flora. We listened to experts, heard grim prognoses. I remember thinking how I grew up in the shadow of the cold war and mutually assured destruction: now it was botanists and biologists, not just zealots and republicans, talking about the end of days.

As our little committee sat in a paradise discussing oblivion, we thought about other committees, specifically the G8 world leaders on that mountainside in Japan in 2008, planting eight trees in eight plots, before retiring to an eight-course banquet that included "corn-stuffed caviar", "diced fatty flesh of tuna fish, avocado and jellied soy sauce", "smoked salmon and sea urchin 'pain surprise style'", and "kelp-flavoured cold Kyoto beef shabu-shabu on asparagus dressed with sesame cream". It became clear we were going to have a Great Committee in our opera, and that I was going to seek out the grossest, most pretentious-sounding meals and set them to music. The last meal on Earth, served to a committee.

A committee of what, though? The first image I had for the opera was of people in suits, with clipboards, sitting around a table talking, while the stars in the window are falling upwards. I believe my notion of perpetual descent originated where many brilliantly half-baked ideas originate, in Doctor Who – but the old cardboard one, not the new hi-tech Bafta-winning export. In an episode remembered from childhood, some henchman topples into a black hole, doomed to fall down it for eternity.

Of course, the grown-up, literary version of this is Milton's Satan and the rebel angels, hurled by God "to bottomless perdition". But we know about those angels – Beelzebub, Moloch and the gang – because Milton had them land in a sulphurous swamp and set his tale in motion. I was interested in seven other angels, who eluded Milton, fell past the swamp, beyond the book, who kept falling, helplessly, forgotten by God, and have been plummeting through space ever since, until they land on a bleak, charred planet, and wonder who they are, what they did wrong, and what brought them to this lonely spot. That sounded close enough to life on Earth, and I had my story.

William Blake famously considered Milton "of the Devil's party without knowing it" – meaning he couldn't help but make the conflicted, flawed and fascinating Satan the hero of Paradise Lost, in contrast to a dull, unbending God and yea-saying angels. But its last lines are unbearably moving, and surely place us humans at the heart of it all: "They hand in hand with wand'ring steps and slow,/ Through Eden took their solitary way."

So a man and a woman, hand in hand, are at the heart of our opera, called Seven Angels. They begin as angels, but take on roles in a fairytale: a waitress, and a gluttonous prince who falls in love with her face reflected in his licked-clean silver plate. And, while the other five angels abandon the once-again despoiled planet to resume their eternal falling, these two struggle to stand on the Earth, recall their humanity, do something. Those who believe we humans truly are "solitary", that the world is all that's before us, have only this to go on, but it's everything. Helpfully to stand or helplessly to fall is a choice made every day.

Only last month, Sarah Palin, a candidate for the most powerful throne on Earth, mounted a huge infernal machine and told an ecstatic crowd she "loved that smell of those emissions!" Now there's a moment that would grace any opera – and I'm sure I read it first in Paradise Lost.